When two user equipment terminals (e.g., mobile communication devices) of a cellular network or other telecommunication system communicate with each other, their data path typically goes through the operator network. The data path through the network may include base stations and/or gateways. If the devices are in close proximity with each other, their data path may be routed locally through a local base station. In general, communications between a network node such as a base station and a wireless terminal is known as “WAN” or “Cellular communication”.
It is also possible for two user equipment terminals in close proximity to each other to establish a direct link without the need to go through a base station. Telecommunications systems may use or enable device-to-device (“D2D”) communication, in which two or more user equipment terminals directly communicate with one another. In D2D communication, voice and data traffic (referred to herein as “communication signals” or “communications”) from one user equipment terminal to one or more other user equipment terminals may not be communicated through a base station or other network control device of a telecommunication system. “Device-to-device (“D2D”) communication may also be known as “sidelink direct” communication (e.g., sidelink communication), or even as “sidelink”, “SL”, or “SLD” communication.
D2D or sidelink direct communication can be used in networks implemented according to any suitable telecommunications standard. A non-limiting example of such as standard is the 3rd Generation Partnership Project (“3GPP”) Long Term Evolution (“LTE”). The 3GPP standard is a collaboration agreement that aims to define globally applicable technical specifications and technical reports for third and fourth generation wireless communication systems. The 3GPP may define specifications for next generation mobile networks, systems, and devices.
Currently 3GPP is specifying a new feature for Rel-14 that covers use cases and potential requirements for LTE support for vehicular communications services (represented by the term, Vehicle-to-Everything (V2X) Services). The feature is documented in the TR 22.885 on LTE Study on LTE Support for V2X Services. Contemplated V2X services may include one or more of the following:                V2V: covering LTE-based communication between vehicles.        V2P: covering LTE-based communication between a vehicle and a device carried by an individual (e.g. handheld terminal carried by a pedestrian, cyclist, driver or passenger).        V2I: covering LTE-based communication between a vehicle and a roadside unit. A roadside unit (RSU) is a transportation infrastructure entity (e.g. an entity transmitting speed notifications).        
Thus far 3GPP deliberations concerning synchronization for vehicle-to-vehicle (V2V) communications have essentially assumed reuse of LTE sidelink for V2V, e.g., assumed that the V2V communications will essentially be indistinct from sidelink direct communications in the access stratum (AS), e.g., may use the same PC5 radio access interface. As such, it has generally been assumed that the LTE 3GPP Rel-12 and Rel-13 D2D synchronization design for SLD would be reused as much as possible. On the other hand, there are still numerous differences between V2X and D2D, such as higher V2X user equipment (UE) density and much higher V2X UE velocity.
In D2D sidelink communications, when a UE is within E-UTRAN coverage, a base station (e.g, eNB) may instruct a UE to become a synchronization source to transmit a sidelink direct synchronization signal (SLSS). On the other hand, if there is no eNB configuration, the UE may by itself become a synchronization source to transmit SLSS once the measured RSRP of the cell is below some pre-configured threshold. On the other hand, when a SLD UE is out-of-coverage, if the received signals from another UE are below some pre-configured S-RSRP threshold, the UE can by itself become a synchronization source to transmit SLSS as well
With the advent of GNSS (Global Navigation Satellite System) and GNSS-equivalent timing sources), a UE participating in V2X communications may be synchronized to GNSS timing even when being outside of E-UTRAN coverage if the GNSS signal received by the UE is sufficiently reliable enough. Meanwhile, given the fact that generally UEs in V2X communication have much higher density than SLD UEs, it is important to control V2X synchronization signal transmission so as to avoid occupying too many resources and generating too much interference. Further, whether the velocity of the UE may affect synchronization signal transmission. Moreover, since in at least some contemplate implementations the V2X will also use the same (SLD) PC5 interface for communications, e.g., coexisting D2D and V2X synchronization signal transmission, handling both D2D synchronization signals and V2X synchronization signals may be problematic.
What is needed are methods, apparatus, and/or techniques for controlling synchronization in vehicle (V2X) communications.